I would get her evaluated. It is never too early to get help. She is not behaving like a "typical" kid and there is help early on (age three) in the public schools for differently wired kids. A good way to tell if your kid has a disorder and is differently wired is that they do not respond to normal parenting techniques.j
I prefer neuropsychs. They are very intensive. This is not a parenting issue and it's not your fault. Most of the time reward charts don't work with our differently wired kids. You can try, most of us have, with little success. I hope you are the one who has good success.
I would still get her evaluated so that she can succeed in school. Bright or not, the behavior issues can hamper her in many ways, socially and academically. You can not make her behave because she is atypical. Please seek outside help. The earlier you get her help, the better the outcome. Good luck

(I am assuming you live in the US...this is basically what we do here. Other countries deal with things differently).
Did she have chaotic years in her first three? Stepdad has come at an awfully early age for her. Where is dad? Did she go through a divorce with you, so to speak? How is stepdad with her? Does he hit her? Yell at her? Scare her? He shouldnh't be doing any of those things.
Does she see her father? These can all be factors. Attachment issues can happen if a child has very chaotic infant-three years with many different changes and caregivers and not much stability. Does she ever tantrum or get violent? Insecure attachment can cause very severe problems in the future, but t he good news is, she is young and there is help. Think about those first three years...they are very important to the wiring in your child's brain.
If your husband is that uncommitted to you and your child, it may work out if he leaves. He does not sound mature or understanding at all. You don't need his threats to leave on top of everything else you have on your plate. Heck, I'm tempted to tell you to show him the door and to tell him not to hit his a*** on the door on the way out. (Sorry)
Hugs and sorry for your scared and hurting mommy heart!